[pixiv] move 'novel' extractors to a 'pixiv-novel' category (#7746)

TODO:
- restore full 'include' functionality
- allow remapping category:subcategory pairs

gallery_dl/config.py

@@ -175,6 +175,7 @@ def remap_categories():
             ("naver"       , "naver-blog"),
             ("chzzk"       , "naver-chzzk"),
             ("naverwebtoon", "naver-webtoon"),
+            ("pixiv"       , "pixiv-novel"),
         )
     elif not cmap:
         return